Enhancement: Add bitrot detection to `diff` command

The output of the `diff` command now includes the modifier `?` for files
to indicate bitrot in backed up files. It will appear whenever there is a
difference in content while the metadata is exactly the same. Since files with
unchanged metadata are normally not read again when creating a backup, the
detection is only effective if the right-hand side of the diff has been created
with "backup --force".

https://github.com/restic/restic/issues/805
https://github.com/restic/restic/pull/4526
